For purposes of clarity, the term “conventional engine” as used in the present application refers to an internal combustion engine wherein all four strokes of the well-known Otto cycle (i.e., the intake, compression, expansion and exhaust strokes) are contained in each piston/cylinder combination of the engine. Also, for purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine as referred to herein comprises:
a crankshaft rotatable about a crankshaft axis;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; and
a crossover passage (port) interconnecting the compression and expansion cylinders, the crossover passage including at least a crossover expansion (XovrE) valve disposed therein, but more preferably including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween.
U.S. Pat. No. 6,543,225 granted Apr. 8, 2003 to Scuderi and U.S. Pat. No. 6,952,923 granted Oct. 11, 2005 to Branyon et al., both of which are incorporated herein by reference, contain an extensive discussion of split-cycle and similar-type engines. In addition, these patents disclose details of prior versions of an engine of which the present disclosure details further developments.
Split-cycle air hybrid engines combine a split-cycle engine with an air reservoir and various controls. This combination enables a split-cycle air hybrid engine to store energy in the form of compressed air in the air reservoir. The compressed air in the air reservoir is later used in the expansion cylinder to power the crankshaft.
A split-cycle air hybrid engine as referred to herein comprises:
a crankshaft rotatable about a crankshaft axis;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft;
a crossover passage (port) interconnecting the compression and expansion cylinders, the crossover passage including at least a crossover expansion (XovrE) valve disposed therein, but more preferably including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween; and
an air reservoir operatively connected to the crossover passage and selectively operable to store compressed air from the compression cylinder and to deliver compressed air to the expansion cylinder.
U.S. Pat. No. 7,353,786 granted Apr. 8, 2008 to Scuderi et al., which is incorporated herein by reference, contains an extensive discussion of split-cycle air hybrid and similar-type engines. In addition, this patent discloses details of prior hybrid systems of which the present disclosure details further developments.
Referring to FIG. 1, an exemplary prior art split-cycle air hybrid engine is shown generally by numeral 10. The split-cycle air hybrid engine 10 replaces two adjacent cylinders of a conventional engine with a combination of one compression cylinder 12 and one expansion cylinder 14. The four strokes of the Otto cycle are “split” over the two cylinders 12 and 14 such that the compression cylinder 12, together with its associated compression piston 20, perform the intake and compression strokes, and the expansion cylinder 14, together with its associated expansion piston 30, perform the expansion and exhaust strokes. The Otto cycle is therefore completed in these two cylinders 12, 14 once per crankshaft 16 revolution (360 degrees CA) about crankshaft axis 17.
During the intake stroke, intake air is drawn into the compression cylinder 12 through an intake port 19 disposed in the cylinder head 33. An inwardly-opening (opening inward into the cylinder and toward the piston) poppet intake valve 18 controls fluid communication between the intake port 19 and the compression cylinder 12.
During the compression stroke, the compression piston 20 pressurizes the air charge and drives the air charge into the crossover passage (or port) 22, which is typically disposed in the cylinder head 33. This means that the compression cylinder 12 and compression piston 20 are a source of high pressure gas to the crossover passage 22, which acts as the intake passage for the expansion cylinder 14. In some embodiments two or more crossover passages 22 interconnect the compression cylinder 12 and the expansion cylinder 14.
The volumetric (or geometric) compression ratio of the compression cylinder 12 of the split-cycle engine 10 (and for split-cycle engines in general) is herein referred to as the “compression ratio” of the split-cycle engine. The volumetric (or geometric) compression ratio of the expansion cylinder 14 of the split-cycle engine 10 (and for split-cycle engines in general) is herein referred to as the “expansion ratio” of the split-cycle engine. The volumetric compression ratio of a cylinder is well known in the art as the ratio of the enclosed (or trapped) volume in the cylinder (including all recesses) when a piston reciprocating therein is at its bottom dead center (BDC) position to the enclosed volume (i.e., clearance volume) in the cylinder when said piston is at its top dead center (TDC) position. Specifically for split-cycle engines as defined herein, the compression ratio of a compression cylinder is determined when the XovrC valve is closed. Also specifically for split-cycle engines as defined herein, the expansion ratio of an expansion cylinder is determined when the XovrE valve is closed.
Due to very high volumetric compression ratios (e.g., 20 to 1, 30 to 1, 40 to 1, or greater) within the compression cylinder 12, an outwardly-opening (opening outwardly away from the cylinder and piston) poppet crossover compression (XovrC) valve 24 at the crossover passage inlet 25 is used to control flow from the compression cylinder 12 into the crossover passage 22. Due to very high volumetric compression ratios (e.g., 20 to 1, 30 to 1, 40 to 1, or greater) within the expansion cylinder 14, an outwardly-opening poppet crossover expansion (XovrE) valve 26 at the outlet 27 of the crossover passage 22 controls flow from the crossover passage 22 into the expansion cylinder 14. The actuation rates and phasing of the XovrC and XovrE valves 24, 26 are timed to maintain pressure in the crossover passage 22 at a high minimum pressure (typically 20 bar or higher at full load) during all four strokes of the Otto cycle.
At least one fuel injector 28 injects fuel into the pressurized air at the exit end of the crossover passage 22 in correspondence with the XovrE valve 26 opening, which occurs shortly before the expansion piston 30 reaches its top dead center position. The air/fuel charge enters the expansion cylinder 14 shortly after the expansion piston 30 reaches its top dead center position. As the piston 30 begins its descent from its top dead center position, and while the XovrE valve 26 is still open, a spark plug 32, which includes a spark plug tip 39 that protrudes into the cylinder 14, is fired to initiate combustion in the region around the spark plug tip 39. Combustion is initiated while the expansion piston is between 1 and 30 degrees CA past its top dead center (TDC) position. More preferably, combustion is initiated while the expansion piston is between 5 and 25 degrees CA past its top dead center (TDC) position. Most preferably, combustion is initiated while the expansion piston is between 10 and 20 degrees CA past its top dead center (TDC) position. Additionally, combustion may be initiated through other ignition devices and/or methods, such as with glow plugs, microwave ignition devices, or through compression ignition methods.
During the exhaust stroke, exhaust gases are pumped out of the expansion cylinder 14 through an exhaust port 35 disposed in the cylinder head 33. An inwardly-opening poppet exhaust valve 34, disposed in the inlet 31 of the exhaust port 35, controls fluid communication between the expansion cylinder 14 and the exhaust port 35. The exhaust valve 34 and the exhaust port 35 are separate from the crossover passage 22. That is, the exhaust valve 34 and the exhaust port 35 do not make contact with the crossover passage 22.
With the split-cycle engine concept, the geometric engine parameters (i.e., bore, stroke, connecting rod length, volumetric compression ratio, etc.) of the compression and expansion cylinders 12, 14 are generally independent from one another. For example, the crank throws 36, 38 for the compression cylinder 12 and the expansion cylinder 14, respectively, may have different radii and may be phased apart from one another such that top dead center (TDC) of the expansion piston 30 occurs prior to TDC of the compression piston 20. This independence enables the split-cycle engine 10 to potentially achieve higher efficiency levels and greater torques than typical four-stroke engines.
The geometric independence of engine parameters in the split-cycle engine 10 is also one of the main reasons why pressure is maintained in the crossover passage 22 as discussed earlier. Specifically, the expansion piston 30 reaches its top dead center position prior to the compression piston reaching its top dead center position by a discreet phase angle (typically between 10 and 30 crank angle degrees). This phase angle, together with proper timing of the XovrC valve 24 and the XovrE valve 26, enables the split-cycle engine 10 to maintain pressure in the crossover passage 22 at a high minimum pressure (typically 20 bar absolute or higher during full load operation) during all four strokes of its pressure/volume cycle. That is, the split-cycle engine 10 is operable to time the XovrC valve 24 and the XovrE valve 26 such that the XovrC and XovrE valves are both open for a substantial period of time (or period of crankshaft rotation) during which the expansion piston 30 descends from its TDC position towards its BDC position and the compression piston 20 simultaneously ascends from its BDC position towards its TDC position. During the period of time (or crankshaft rotation) that the crossover valves 24, 26 are both open, a substantially equal mass of gas is transferred (1) from the compression cylinder 12 into the crossover passage 22 and (2) from the crossover passage 22 to the expansion cylinder 14. Accordingly, during this period, the pressure in the crossover passage is prevented from dropping below a predetermined minimum pressure (typically 20, 30, or 40 bar absolute during full load operation). Moreover, during a substantial portion of the intake and exhaust strokes (typically 90% of the entire intake and exhaust strokes or greater), the XovrC valve 24 and XovrE valve 26 are both closed to maintain the mass of trapped gas in the crossover passage 22 at a substantially constant level. As a result, the pressure in the crossover passage 22 is maintained at a predetermined minimum pressure during all four strokes of the engine's pressure/volume cycle.
For purposes herein, the method of opening the XovrC 24 and XovrE 26 valves while the expansion piston 30 is descending from TDC and the compression piston 20 is ascending toward TDC in order to simultaneously transfer a substantially equal mass of gas into and out of the crossover passage 22 is referred to herein as the Push-Pull method of gas transfer. It is the Push-Pull method that enables the pressure in the crossover passage 22 of the split-cycle engine 10 to be maintained at typically 20 bar or higher during all four strokes of the engine's cycle when the engine is operating at full load.
As discussed earlier, the exhaust valve 34 is disposed in the exhaust port 35 of the cylinder head 33 separate from the crossover passage 22. The structural arrangement of the exhaust valve 34 not being disposed in the crossover passage 22, and therefore the exhaust port 35 not sharing any common portion with the crossover passage 22, is preferred in order to maintain the trapped mass of gas in the crossover passage 22 during the exhaust stroke. Accordingly, large cyclic drops in pressure, which may force the pressure in the crossover passage below the predetermined minimum pressure, are prevented.
The XovrE valve 26 opens shortly before the expansion piston 30 reaches its top dead center position. At this time, the pressure ratio of the pressure in the crossover passage 22 to the pressure in the expansion cylinder 14 is high, due to the fact that the minimum pressure in the crossover passage is typically 20 bar absolute or higher and the pressure in the expansion cylinder during the exhaust stroke is typically about one to two bar absolute. In other words, when the XovrE valve 26 opens, the pressure in the crossover passage 22 is substantially higher than the pressure in the expansion cylinder 14 (typically in the order of 20 to 1 or greater). This high pressure ratio causes initial flow of the air and/or fuel charge to flow into the expansion cylinder 14 at high speeds. These high flow speeds can reach the speed of sound, which is referred to as sonic flow. This sonic flow is particularly advantageous to the split-cycle engine 10 because it causes a rapid combustion event, which enables the split-cycle engine 10 to maintain high combustion pressures even though ignition is initiated while the expansion piston 30 is descending from its top dead center position.
The split-cycle air-hybrid engine 10 also includes an air reservoir (tank) 40, which is operatively connected to the crossover passage 22 by an air reservoir tank valve 42. Embodiments with two or more crossover passages 22 may include a tank valve 42 for each crossover passage 22, which connect to a common air reservoir 40, or alternatively each crossover passage 22 may operatively connect to separate air reservoirs 40.
The tank valve 42 is typically disposed in an air tank port 44, which extends from the crossover passage 22 to the air tank 40. The air tank port 44 is divided into a first air tank port section 46 and a second air tank port section 48. The first air tank port section 46 connects the air tank valve 42 to the crossover passage 22, and the second air tank port section 48 connects the air tank valve 42 to the air tank 40.
The volume of the first air tank port section 46 includes the volume of all additional recesses which connect the tank valve 42 to the crossover passage 22 when the tank valve 42 is closed. Preferably, the volume of the first air tank port section 46 is small (e.g., less than approximately 20%) relative to the volume of the crossover passage 22. More preferably, the first air tank port section 46 is substantially non-existent, that is, the tank valve 42 is most preferably disposed such that it is flush against the outer wall of crossover passage 22.
The tank valve 42 may be any suitable valve device or system. For example, the tank valve 42 may be a pressure-activated check valve, or an active valve which is activated by various valve actuation devices (e.g., pneumatic, hydraulic, cam, electric or the like). Additionally, the tank valve 42 may comprise a tank valve system with two or more valves actuated with two or more actuation devices.
The air tank 40 is utilized to store energy in the form of compressed air and to later use that compressed air to power the crankshaft 16, as described in aforementioned U.S. Pat. No. 7,353,786 to Scuderi et al. This mechanical means for storing potential energy provides numerous potential advantages over the current state of the art. For instance, the split-cycle engine 10 can potentially provide many advantages in fuel efficiency gains and NOx emissions reduction at relatively low manufacturing and waste disposal costs in relation to other technologies on the market such as diesel engines and electric-hybrid systems.
The air hybrid split-cycle engine 10 can be run in a normal operating mode (referred to as the engine firing (EF) mode or as the normal firing (NF) mode) and four basic air hybrid modes. In the EF mode, the engine 10 functions normally as previously described in detail herein, operating without the use of its air tank 40. In the EF mode, the tank valve 42 remains closed to isolate the air tank 40 from the basic split-cycle engine 10.
In the four hybrid modes, the engine 10 operates with the use of its air tank 40. The four hybrid modes are:
1.Air Expander (AE) mode, which includes using compressed air energy from the air tank 40 without combustion;
2.Air Compressor (AC) mode, which includes storing compressed air energy into the air tank 40 without combustion;
3.Air Expander and Firing (AEF) mode, which includes using compressed air energy from the air tank 40 with combustion; and
4.Firing and Charging (FC) mode, which includes storing compressed air energy into the air tank 40 with combustion.
In the split-cycle engine 10, the compression and expansion cylinders 12, 14 are positioned in-line with each other and share a common cylinder head 33 in which the crossover passage 22 is formed. Additionally, the common head 33 must include several cooling passages (not shown) to enable engine coolant to be pumped through the head 33 to remove heat from the compression cylinder 12, the expansion cylinder 14, and the crossover passage 22. Because the crossover passage 22 is formed integrally with the cylinder head 33, it is very difficult to independently control the temperature of the crossover passage 22 (and the fluid therein) relative to the cylinders 12, 14.
Also, the relative lack of available space in the cylinder head 33 imposes undesirable size and shape restrictions on the crossover passage(s) 22 and the air reservoir control valve(s) 42. For example, the crossover passage 22 or the first air tank port section 46, which connects the valve 42 to the crossover passage 22, may have to be curved in order to avoid breaking through or getting too close to the various cooling passages. The curved crossover passages would then be longer than necessary, which would increase heat losses therein and decrease efficiency. The curved first tank port section 46 would undesirably combine with the volume of the crossover passage to decrease pressure in the crossover passage and also decrease efficiency. Moreover, the common head may become so crowded that it may become very difficult (if not virtually impossible) to connect a tank valve 42 to the crossover passage 22 without breaking through or coming too close to some of the cooling passages.
Still further, the casting process that is typically used to form the crossover passage 22 in the cylinder head 33 leaves behind manufacturing artifacts that disrupt air flow in the crossover passage 22 and undesirably limit the shape and size of the crossover passage(s) 22. Accordingly, there is a need for improved split-cycle engine configurations.